(1) Field of the Invention
The present invention relates to obtaining target location information, and more specifically to systems and methods for obtaining target elevation information.
(2) Description of the Prior Art
Systems and methods for estimating the heights of objects, or the changes in the elevation plane of an image, to ultimately produce three-dimensional imagery have been known in the RADAR and SONAR communities for a number of years.
Basically, the known systems and methods use differences in path lengths between signals received at two or more known vertical locations to infer elevation changes in the terrain and the height of objects. One method commonly used in the RADAR community for this type of topographic imaging is interferometry, where phase differences of the received signals are used to estimate the path length differences and ultimately infer elevation changes.
Interferometer methods or techniques are broadly categorized as either multi-pass or single-pass. Multi-pass techniques use the relative separation between multiple trajectories or passes of a platform with a single sensor to form the interferometer. Single-pass techniques use the separation of two or more receivers carried by the same platform to form the interferometer.
Multi-pass methods typically are not used in aircrafts or underwater vehicles due to the complexities and inefficiencies associated with accurately duplicating trajectories and motions between multiple passes. Even if the trajectories and motions can be duplicated, changes in environmental conditions, in the scene itself, or in a combination of the two, during the time between passes can compromise the results.
More recently, interferometer topographic mapping methods have been combined with synthetic aperture (SA) imaging methodologies to generate high-resolution three-dimensional imagery. Such systems commonly are referred to as Interferometric Synthetic Aperture Radar (InSAR) and Interferometric Synthetic Aperture Sonar (InSAS) in the RADAR and SONAR communities, respectively.
The systems and methods discussed hereinafter are presented and developed principally from an InSAS perspective. Those of skill in the art will recognize that the systems and methods disclosed herein are equally applicable to InSAR. Thus, references to InSAS systems and methods can be taken also to include InSAR systems and methods.
Current single-pass InSAS systems and methods rely on a single transmitter and a multi-element receiver array to estimate changes in the elevation plane. Referring to prior art FIG. 1, there is shown a schematic representation of such a system S. Transmitter TX and two vertical receivers, RX1 and RX2, are spaced along vertical axis Y. Transmitter TX and receivers RX1 and RX2 are taken to be single elements of length L along fore-aft axis Z (towards and away from the viewer of FIG. 1).
With knowledge of the altitude H of receiver RX1, the elevation or height h of point A can be approximated by:h=H−r1 cos (θ), where  (1)
r1 is the path length between point A and receiver RX1, and
θ is the angle of the backscattered signals.
With knowledge of the baseline separation D between receivers RX1 and RX2 and the tilt angle β of receivers RX1 and RX2, θ can be estimated via trigonometric manipulations with inference of the path lengths r1 and r2, where r2 is the path length between point A and receiver RX2. As shown in FIG. 1, the angle is defined as ψ=π−β−θ. Thus:θ=π−ψ−β,
                                          θ            =                          π              -                              arccos                ⁡                                  [                                                                                    D                        2                                            +                                              r                        1                        2                                            -                                              r                        2                        2                                                                                    2                      ⁢                                              Dr                        1                                                                              ]                                            -              β                                ,          and                ⁢                                  ⁢                  θ          =                                    arccos              ⁡                              [                                                                            r                      2                      2                                        -                                          D                      2                                        -                                          r                      1                      2                                                                            2                    ⁢                                          Dr                      1                                                                      ]                                      -                          β              .                                                          (        2        )            
The path lengths r1 and r2 are related by:r2=r1+dr=r1+cτ, where  (3)
dr is the difference in path lengths r1 and r2,
c is the propagation speed of sound, and
τ is the time of arrival difference between the signals at receivers RX1 and RX2.
Time of arrival differences are estimated via known signal cross correlation methods in either the time or frequency domain. The performance of the chosen cross correlation method is enhanced by improving the signal-to-noise ratio as well as improving the resolution of the range and along-track position of the cell or pixel (point A) for which elevation information is being inferred.
Fine along-track resolutions from relatively small physical arrays are obtained using known SAS imaging techniques, which exploit the forward motion of the platform to synthesize a larger array aperture than that of the real or physical array aperture. Multi-element receiver or vernier array SAS systems are capable of achieving fine resolutions and increasing the area coverage rate (ACR) of the system.
Reducing the width of the elements and increasing the number of elements improve the potential resolution and ACR of the system. Unfortunately, doing so increases the total channel count and receiver hardware complexity of the SAS, a very important factor in the overall system design. Further, an InSAS based on identical, dual, vertical receivers would require as a minimum twice the number of elements, with corresponding increases in the mechanical, electrical and functional complexity of the hardware.
In autonomous underwater vehicle (AUV) systems in particular, where energy is limited, the added complexity of such a design is clearly unwanted. In such systems, it is desirable to minimize the overall power consumption, volume and weight of the AUV so as to optimize the endurance, or operational time, as a function of the available energy.
What are needed are systems and methods for obtaining target elevation information that can maintain the high resolution of multi-element receivers, while reducing the overall power consumption, volume and weight of InSAS systems having dual, vertical receivers.